A molded product of a fiber-reinforced resin, which is increased in strength by including reinforcing fiber, is used for various applications. In order to obtain the molded product by injection molding, a thermoplastic resin is melted by rotating a screw in a cylinder which is a plasticizing device and is kneaded with fiber, and thereafter the resultant is injected into the die of an injection molding device.
In order to obtain an effect of enhancing strength using reinforcing fiber, it is desirable that the reinforcing fiber is uniformly dispersed in a resin. In order to accomplish the uniform dispersion, strict kneading conditions may be employed. However, this conversely causes an increase in shear force in the kneaded product and thus the reinforcing fiber may be cut. Accordingly, the fiber length after molding is significantly reduced compared to the original fiber length, and thus there is concern that the obtained fiber-reinforced resin molded product may not satisfy desired properties (PTL 1). Therefore, it is necessary to select injection molding conditions with which appropriate shear force is obtained during kneading.
In addition, in order to prevent the fiber from being cut in a plasticization process, a plasticizing device provided with a cylinder which includes a supply port for supplying a thermoplastic resin starting material to the cylinder, a supply port for supplying a fiber material, and a vent port for removing gas included in a resin, and a screw is suggested (PTL 2).
The cutting of fiber in the plasticization process of the fiber-reinforced resin is performed mainly in the first half of the plasticizing device, that is, in a supply unit or a compression unit of resin pellets, and thus the supply port of the fiber material in the plasticizing device is provided at a stage after the compression unit is used in which the thermoplastic resin is sufficiently melted.